In general, this invention relates to a system for controlling the exposure time of a shutter of a single lens reflex (SLR) camera. More particularly, it relates to an improved system including a closed-loop circuit arrangement that provides an analog signal used by a timing circuit to control exposure time.
In an SLR camera, light entering the camera through the lens can follow one of two paths. While the photographer is composing the picture, the light is reflected by a mirror toward a focusing screen to form an image for viewing through a viewfinder. When the photographer actuates the shutter button to take a picture, the mirror pivots so that the light is focused at the film plane.
The displacement of the mirror so that it is out of the way of the film has the result that the light does not impinge on the focusing screen while the shutter is being opened and closed. In an SLR camera having a through-the-lens (TTL) exposure control system, the photocell thereof is positioned so as to be responsive to the light entering the camera when the mirror is down, and accordingly neither receives light nor produces a useful signal while the shutter is being opened and closed. Because of this, and further because a timing circuit in the exposure control system requires a tiiming-control signal after the shutter is actuated, it is essential to provide a storage or memory function.
One conventional arrangement for providing this memory function employs a capacitor as a signal storing element. Usually, in an effort to minimize the decay of the voltage stored by the capacitor, a high input impedance field effect transistor stage (FET) is provided as a buffer between the capacitor and the timing circuit.
This conventional arrangement has several disadvantages. In operation, the stored value will vary with time because of leakage currents of the capacitor and the FET. Maufacturing problems also arise, particularly with respect to the selection of elements and strict tolerances therefor, and also with respect to mechanical problems involved in water-proofing of the circuitry involved. Further, there has been a serious problem in prior art cameras in which an ammeter serves as the exposure meter display. In particular, an ammeter is not sufficiently shock resistant, and its reliability may accordingly be deteriorated in use.
Various proposals have heretofore been made to overcome these problems with respect to storage and display. For example, it has been proposed that an analog signal representing exposure time be converted into a corresponding digital value to be stored in flip-flop circuitry, and that this digital value be displayed by a display element such as a light emitting diode, a liquid crystal, or a small electric lamp. Most of the analog-to-digital (A-D) converters thus proposed use counters in which the respective count values are directly associated with the exposure time. With these prior art A-D converters, there have usually been encountered two problems. One of these problems relates to resolving power. With a quantization unit of, for example, one ms, as many as 1000 different intermediate values may be taken between 1S and 2S, whereas no intermediate value may be taken between 1ms and 2ms. The other problem relates to response velocity. The response time is remarkably different relative to variations in exposure time and even at an identical ratio. For example, consider the variation from 1ms to 2ms and the variation from 1S to 2S. The response time relative to the latter case is 1000 times as long as the response time relative to the former case. Furthermore, a complex decoder has usually been required in such a circuit to obtain the output necessary for display. It should be noted here that, although it would be possible to take intermediate values even between 1ms and 2ms, by increasing the number of stages the counter, whereby the unit of quantization is made more fine, the previously mentioned problems remain unsolved and the circuit arrangement is correspondingly complicated.